The present disclosure relates generally to airborne devices that convert energy during a crosswind power generating phase, and related wind energy conversion systems and methods.
To take advantage of the greater force of winds available at different altitudes, airborne devices have been developed that work at altitudes above the heights currently reached by ground-based wind turbines. Some of the existing airborne systems use tethered devices to generate power by flying into the wind or by flying crosswind.
Devices that fly into the wind typically have flight controls for two axes, i.e., one axis for pitch and one axis for roll, to keep the main wing parallel to the ground while the tether is let out from a ground winching device at a controlled speed. The wind forces the wing up creating a resulting force that is transmitted down the tether to a generator, to pump water, to turn a flywheel, or other energy conversion actions.
A crosswind flight path typically refers to a path followed by an airborne device wherein the wind has a component roughly perpendicular to the direction of travel, that is, the flight path can be separated into two components, a crosswind component and a headwind or tailwind component.
A crosswind motion of a wing is generally more efficient than a downwind motion because a crosswind motion allows the wing to fly many times the speed of the wind and harvest energy from an area that is many times larger than the area of the wing. When a wing moves in a plane normal to the wind vector, the apparent wind velocity becomes several times larger than the wind velocity itself. The crosswind motion generally results in higher forces generated in the tether compared to flying into the wind. Generally, crosswind kite speed is a function of the prevailing wind speed times the airborne device's net lift to drag ratio. However, existing crosswind devices typically require more complex control systems than devices that fly into the wind. For example, crosswind devices usually require controls for three axes, i.e., pitch, roll, and yaw, to follow a more complex fight pattern, such as a circular or figure eight pattern, during the energy generating portion of the flight path.
Known devices that use crosswind motion to increase force in a tether are described, for example, in U.S. Pat. No. 3,987,987 and in an article by Miles L. Loyd “Crosswind Kite Power,” published in the Journal of Energy, Vol. 4, No. 3. May-June 1980. Variations of airborne wind energy conversion systems with a generator on the ground have been described. For example, U.S. Pat. No. 6,072,245 describes multiple airfoils that are connected to a single generator. The complete disclosures of the above patents are herein incorporated by reference for all purposes.
Known wind energy conversion systems, however, are not entirely satisfactory for the range of applications in which they are employed. For example, existing airborne wind energy conversion devices typically require complex systems using devices with a large turn radius when flying crosswind and consequently require a large surface area for operation.
Thus, there exists a need for airborne wind energy conversion systems, devices and methods that improve upon and advance the design of known devices, systems and methods.